The huge volume of water contained in the oceans (and seas), 137 × 107 cubic km (about 33 × 107 cubic miles), has been produced during Earth’s geologic history. There is little information on the early history of Earth’s waters. However, fossils dated from the Precambrian some 3.3 billion years ago show that bacteria and cyanobacteria (blue-green algae) existed then, indicating the presence of water during that period. Carbonate sedimentary rocks, obviously laid down in an aquatic environment, have been dated to 1 billion years ago. Also, there is fossil evidence of primitive marine algae and invertebrates from the Ediacaran Period (635 million to 541 million years ago).
The presence of water on Earth at even earlier times is not documented by physical evidence. It has been suggested, however, that the early hydrosphere formed in response to condensation from the early atmosphere. The ratios of certain chemical elements on Earth indicate that the planet formed by the accumulation of cosmic dust and was slowly warmed by radioactive and compressional heating. This heating led to the gradual separation and migration of materials to form Earth’s core, mantle, and crust. The early atmosphere is thought to have been highly reducing and rich in gases, notably in hydrogen, and to include water vapour.
Earth’s surface temperature and the partial pressures of the individual gases in the early atmosphere affected the atmosphere’s equilibration with the terrestrial surface. As time progressed and the planetary interior continued to warm, the composition of the gases escaping from within Earth gradually changed the properties of its atmosphere, producing a gaseous mixture rich in carbon dioxide (CO2), carbon monoxide (CO), and molecular nitrogen (N2). Photodissociation (i.e., separation due to the energy of light) of water vapour into molecular hydrogen (H2) and molecular oxygen (O2) in the upper atmosphere allowed the hydrogen to escape and led to a progressive increase of the partial pressure of oxygen at Earth’s surface. The reaction of this oxygen with the materials of the surface gradually caused the vapour pressure of water vapour to increase to a level at which liquid water could form. This water in liquid form accumulated in isolated depressions of Earth’s surface, forming the nascent oceans. The high carbon dioxide content of the atmosphere at this time would have allowed a buildup of dissolved carbon dioxide in the water and made these early oceans acidic and capable of dissolving surface rocks that would add to the water’s salt content. Water must have evaporated and condensed rapidly and accumulated slowly at first. The required buildup of atmospheric oxygen was slow because much of this gas was used to oxidize methane, ammonia, and exposed rocks high in iron. Gradually, the partial pressure of the oxygen gas in the atmosphere rose as photosynthesis by bacteria and photodissociation continued to supply oxygen. Biological processes involving algae increased, and they gradually decreased the carbon dioxide content and increased the oxygen content of the atmosphere until the oxygen produced by biological processes outweighed that produced by photodissociation. This, in turn, accelerated the formation of surface water and the development of the oceans.
Alyn C. Duxbury The Editors of Encyclopaedia Britannica